Optical sensor for focusing control

Abstract

An optical sensor for delivering error signals in order to control focusing of a lens on a movable recording medium or optical memory and more particularly a videodisk comprises a laser for producing a beam which is focused at a point S 1 . The divergent beam which proceeds from the point S 1 is reflected from the face AB of a prism and is focused at a point S 2 . If the point S 2 is located in the plane of the movable recording medium or so-called memory plane, the beam reflected from the memory plane is autocollimated and passes back through the focusing lens. A part of the beam enters the prism through the face AB, then emerges through the face AC and a circular light spot is formed in the plane corresponding to the circle of least confusion. If the point S 2 is not located in the memory plane, the light spot is deformed and these deformations are detected by four juxtaposed cells placed in the plane of the circle of least confusion.

What is claimed is: 
     
       1. In an optical recording system including a movable recording medium, an optical source and a lens for focusing an optical beam from said optical source on said recording medium, means for controlling the position of said lens, said means for controlling comprising: refracting means having first and second planar faces, said first planar face being positioned for reflecting said optical beam, said reflected beam passing through said lens for focusing on said recording medium, a portion of said reflected and focused beam being reflected back through said lens and transmitted through said refracting means, said refracting means being arranged to astigmatize said reflected and focused beam to form an astigmatized beam defining a circle of least confusion;   photoelectric means for detecting in a first plane an elongation of a spot projected on said first plane by said astigmatized beam and outputting an electric a signal representative of a degree and direction of said elongation.   
     
     
       2. The system of claim 1 wherein said refracting means is a prism of refracting material. 
     
     
       3. The system of claim 1 wherein said means for detecting comprising four juxtaposed photoelectric cells positioned in said first plane; and servo means for moving said lens in response to a value of said electric signal, so as to change a lens-to-recording medium distance.   
     
     
       4. The system of claim 1, wherein the movable recording medium is a video disk. 
     
     
       5. The system of claim 1, wherein the four cells have a substantially square shape and are juxtaposed in order to form a second square, wherein diagonally opposite cells of the second square are connected to separate adding circuits, and wherein said adding circuits are connected to a differential amplifier for providing a correction signal and said amplifier is connected to the servo means for adjusting the lens-to-recording medium distance. 
     
     
       6. The system of claim 1, wherein the optical source is a laser.